1. Field of the Invention
The present invention relates to an engine exhaust emission purification apparatus for reducing and purifying nitrogen oxides (NOx) in an exhaust emission by using a liquid reducing agent (hereafter referred to as “exhaust emission purification apparatus”) and particularly to a technology for rarely causing clogging of an injection hole of an injection nozzle for the liquid reducing agent.
2. Description of the Related Art
As an exhaust emission purification system for purifying NOx included in an engine exhaust emission, there has been proposed an exhaust emission purification apparatus as disclosed in Japanese Patent Application Laid-open No. 2000-27627.
In this exhaust emission purification apparatus, a reduction catalytic converter is disposed in an exhaust system of the engine, a reducing agent is supplied by injection to an exhaust emission at a position upstream of the reduction catalytic converter, and NOx in the exhaust emission is subjected to reaction with the reduction agent in the reduction catalytic converter for purifying the exhaust emission by converting the NOx into harmless components. The reducing agent is stored in a storage tank in a liquid state at room temperature and is injected and supplied from an injection nozzle in a required amount corresponding to an operating state of the engine. At this stage, as the reducing agent, a liquid reducing agent such as a urea aqueous solution, an ammonia aqueous solution, and diesel oil having hydrocarbon as main components is used.
However, with the conventional exhaust emission purification apparatus, the injection hole of the injection nozzle might be clogged during the supply of the liquid reducing agent by injection and the injection for supplying the liquid reducing agent will become impossible in some cases. As a result, the reduction reaction of NOx in the reduction catalytic converter would not proceed to result in emission of the NOx before it is purified and therefore, it might become impossible to obtain a required performance for the exhaust emission purification. It is assumed that this problem is caused by such a phenomenon that only a solvent evaporates and the dissolved matter of the liquid reducing agent is precipitated in the injection nozzle when a temperature of the injection nozzle increases under an influence of exhaust heat and a temperature of the liquid reducing agent increases to or over a boiling point of the solvent.
In the injection nozzle, when the temperature of the liquid reducing agent increases to or over the boiling point of the solvent and the dissolved matter is precipitated and when the temperature further increases over a melting point of the dissolved matter, the dissolved matter melts and therefore the clogging of the injection hole is expected to be cancelled.
On the other hand, at a portion less susceptible to the heat of exhaust emission, e.g., piping for supplying the liquid reducing agent to the injection nozzle, a temperature increases to or over the boiling point of the solvent but may not increase to or over the melting point of the dissolved matter. In this case, the dissolved matter remains precipitated in the injection nozzle and the piping for the nozzle, and therefore it becomes impossible to make an injection supply of the liquid reducing agent from the injection nozzle. As a result, exhaust emission purification by the reduction catalytic converter may become insufficient to resultantly cause emission of a large amount of NOx. Such a problem becomes prominent especially when the injection supply of the liquid reducing agent stop under a condition where the temperatures of the injection nozzle and the piping for the nozzle equal to or higher than the boiling point of the solvent or when an amount of liquid reducing agent supplied by injection is small.